NEW YORK -- As the match wore on Monday night and the upset was looking more inevitable, the crowd on Arthur Ashe Stadium began to lose its intensity.

Roger Federer's coach, Ivan Ljubicic, sat still, fluttering his lips as Federer racked up another error. Federer's wife, Mirka, spoke for a lot of us, as she scratched her head when Federer dropped the third set.

One of the many fervent Federer fans who was leading a wild cheer for his man early in the match stared wistfully into the New York air, unable to comprehend the sadness.

In what initially appeared to be another seamless performance, Federer squandered a one-set lead and fell to No. 55 John Millman 3-6, 7-5, 7-6 (7), 7-6 (3) in the fourth round of the US Open.

John Millman? Yes, that John Millman, an Aussie journeyman who had never made a quarterfinal of a Grand Slam.

Judging by the way he played and the energy he showed Monday, it's surprising this is the first we're really hearing of Millman.

So what's his deal anyway? Who is he? Why now? Why against Federer? Here are a few quick-hitting things about the guy responsible for the biggest upset on the men's side of the draw:

John Millman owes Roger Federer a lot

John Millman won his first-career match against a top-10 player. AP Photo/Jason DeCrow

Not in terms of money, but more just a hearty thank you. Before the start of the grass-court season, Federer invited Millman to train with him in Switzerland. This likely wasn't the gratitude Federer was hoping for, but after the match, Federer had nothing but praise for Millman.

"He reminds me of David Ferrer and those other guys that, you know, I admire a lot when I see them, when I see how they train, the passion they have for the game," Federer said. "I love his intensity."

During the broadcast, ESPN analyst Brad Gilbert said he received a text from Andy Murray, who said Millman loves the hot, humid conditions like Monday night. Millman spends a good amount of training -- when he's not with Federer in Switzerland -- in Brisbane, Australia, which has similar conditions to the 83 degree, 74 percent humidity of Monday in New York.

Said Millman: "Roger's a hero of mine. I look up to him. I really like his team. He's always been, you know, one of the guys in the locker rooms, we'll always chat, very approachable. I felt a little bit guilty today because he didn't have his best day, and that's for sure."

John Millan was 0-10 in his career against top-10 players

And now he's 1-10. The stat itself shouldn't surprise anyone. Millman is No. 55 now and has never been higher than No. 49. Matter of fact, last October, Millman dropped to No. 218 in the world. He has zero career titles and hardly a name invoked as a threat at even lower-level events. Further, only five other players ranked 50th or lower had ever beaten Federer in a major and none since Sergei Stakhovsky at Wimbledon in 2013.

Said Millman: "Obviously it's probably a bit of a shock to a lot of people [that I won]. But that's a great thing about tennis, that's the great thing about sport. There's always upsets that can happen."

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Federer falls to Millman in four sets

No. 2 Roger Federer wins the first set but loses the next three, including two via tiebreak, in falling to John Millman in the US Open's fourth round.

John Millman joined a small group of players to beat Federer at night at the US Open

This graphic says it all, no?

ESPN Stats & Analysis

Said Millman: "Look, I always was of the opinion that I was in the fourth round for a reason. I've never played anyone's reputation. That's been a constant with me for, you know, ever since I played the game. Because I think if you do that, then you start behind the eight-ball straightaway."

John Millman is almost the best returner at the US Open

After Monday's play, Millman has returned 81 percent of his opponents' serve at this year's US Open. That's tied for first with Novak Djokovic. It just so happens these two are playing each other Wednesday. Altogether, Millman has received 381 serves and safely put 309 into play. His winning percentage on those points is only 28 percent on first serves (Djokovic is 35 percent), but Millman has won nearly 60 percent of return points on his opponents' second delivery.

It helped that Federer made only 49 percent of his first serves, easily the lowest of his four matches here.

But this day was about Millman, his return and his stellar form.

Said Millman: "Hopefully I create a few more memories. I'll obviously remember this for a long, long time. I hope the people who are watching here and back home remember it, too."

John Millman is healthy, and that matters

Millman was out of tennis for nearly a year between May 2013 and April 2014 because of a career-threatening shoulder injury. Then in 2017, he missed the first four months of the season with a torn tendon in his groin that required surgery. Just a few weeks ago, he pulled out his first-round qualifying match in Cincinnati with an ailing back.

Before the tournament started, Millman conceded his confidence was low in an interview with the ATP. He said it was difficult because the field is so deep that even a minor health setback is a major blow.

Still, Millman is 18-15 this season, and there's little question he'll play more matches this year than any other in his professional career.

Said Millman: "That's challenging. It's challenging financially. It's challenging physically. It's challenging mentally. Yeah, it's tough to start all over again a few times. But you do it. And you do all those moments in rehab, you do all that for something like this. It all becomes a little bit more rewarding."